I have a 1992 Volvo 240 wagon with the same issue that it will not shift into O/D on the highway. Having the relay and Solenoid checked tomorrow but have also been concerned about a high pitch whining noise from the shift area that goes higher when you speed up and lower when you slow down. May be time to put a bullet in the grill ! - IanI have a 1992 Volvo 240 wagon with the same issue that it will not shift into O/D on the highway. Having the relay and Solenoid checked tomorrow but have also been concerned about a high pitch whining noise from the shift area that goes higher when you speed up and lower when you slow down. May be time to put a bullet in the grill ! - Ian

my overdrive light never goes out, it stay on constantly and never shifts 2 overdrive when i'm on the highway and the relay makes a humming noise that follows the rpm of the enginemy overdrive light never goes out, it stay on constantly and never shifts 2 overdrive when i'm on the highway and the relay makes a humming noise that follows the rpm of the engine

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Remove glove box via 8 screws / pull out top light switch connector / Inside on left side you will find a Over Drive Relay Switch / Replace Relay switch / Re install in reverse order. Also you should hear the overdrive relay switch come on every time it is energized on or off the car. ( bench with 12 volts fed to it)

The failure of the relay for the overdrive is a common problem with this vehicle. However, before you replace it, the wiring that goes to the overdrive solenoid needs to be checked. It tends to rub on the transmission housing, and then it shorts out the relay. If you replace the relay without fixing this, it will destroy the new relay!

To access the relay, pull the glove box housing, an it's right behind the center console.